extra interfaces provided by this content (this SHOULD NOT
include TP_IFACE_CALL_CONTENT itself). Implementation must first chainup on
parent class implementation then add extra interfaces into the GPtrArray.

optional; virtual method called when user requested to send
a DTMF tone. Note that this method is already implemented by
TpBaseMediaCallContent and so does not have to be overriden when using that
subclass

optional; virtual method called when user requested to stop
sending currently being played DTMF tones. Note that this method is already
implemented by TpBaseMediaCallContent and so does not have to be overriden
when using that subclass

optional; virtual method called when user requested to send
multiple DTMF tones. Note that this method is already implemented by
TpBaseMediaCallContent and so does not have to be overriden when using that
subclass

a specific reason for the change, which may be a D-Bus error in
the Telepathy namespace, a D-Bus error in any other namespace (for
implementation-specific errors), or the empty string to indicate that the
state change was not an error.

message :

an optional debug message, to expediate debugging the potentially
many processes involved in a call.